GaN-based compound semiconductors including the III group element(s) such as Al, Ga and In and the V group element of N have been expected as compound semiconductor materials for light emitting devices or power devices because of their favorable band structures and chemical stability, and application thereof has been attempted. In particular, as a light source for an optical information recording apparatus of next generation, production of a blue semiconductor laser device by stacking a plurality of GaN-based semiconductor layers on a sapphire substrate, for example, has been attempted vigorously.
An example of such a blue semiconductor laser device is shown in FIG. 20, wherein a refractive index difference is caused at the boundary of the ridge-type waveguide to thereby confine light within the waveguide for lasing (see, e.g., Jpn. J. Appl. Phys., Vol. 37 (1998), pp. L309-L312, and Jpn. J. Appl. Phys., Vol. 39 (2000), pp. L647-L650). In this conventional GaN-based laser 2000, a GaN thick film is formed on a (0001) plane sapphire substrate (not shown), followed by removal of the sapphire substrate. Stacked successively on the (0001) plane GaN thick film substrate 2001 are a Si-doped n-type GaN lower contact layer 2003, a Si-doped n-type Al0.1Ga0.9N lower clad layer 2004, a Si-doped n-type GaN lower guide layer 2005, a multiple quantum well active layer 2006 utilizing InxGa1−xN (0≦x≦1), a Mg-doped p-type Al0.2Ga0.8N evaporation-preventing layer 2007, a Mg-doped p-type GaN upper guide layer 2008, a Mg-doped p-type Al0.1Ga0.9N upper clad layer 2009, and a Mg-doped p-type GaN upper contact layer 2010.
At the top of semiconductor laser 2000, a linear ridge stripe 2011 is formed with a portion of upper clad layer 2009 and upper contact layer 2010. The ridge stripe serves to confine a horizontal transverse mode. A dielectric film 2012 of silicon oxide is formed on each side surface of ridge stripe 2011, which serves as a current-constricting layer for introducing electric current only from the top surface of the ridge stripe. The region delimited by the broken line represents the top portion of the ridge stripe in this figure as well as in the other figures.
A p-side electrode 2013 is formed to cover the top of ridge stripe 2011 and dielectric film 2012. Further, an n-side electrode 2014 is deposited on lower contact layer 2003 having been partially exposed by formation of a mesa 2015 having a side surface parallel to ridge stripe 2011. These electrodes serve to provide the power to semiconductor laser 2000.
Resonator end faces are formed by dry etching. The wafer is divided into bars not to break the end faces, and each bar is then divided parallel to the ridge stripe into chips, to thereby obtain GaN-based lasers 2000.
In semiconductor laser 2000, the light confinement is achieved by the stepped refractive index distribution by virtue of ridge stripe portion 2011, so that it is possible to obtain stable lasing of the horizontal transverse mode with a low threshold current. Furthermore, the lifetime of that laser device exceeds 10,000 hours, and thus it is considered the semiconductor laser technology has almost been completed in terms of the long life and accompanying reliability of the device.
The laser having the structure as shown in FIG. 20, however, is known to cause ripples in its emission spectrum. More specifically, in association with a minimum longitudinal mode interval (Fabry-Perot mode interval) λ0 that is determined from the resonator length in the stripe direction, a plurality of modes occur at mode intervals λ1=nλ0 (n is an integer). That is, only some modes among the possible Fabry-Perot modes actually occur.
It is known that a problem of noise arises when such a laser is used. A mode-hopping noise or the like occurs in the laser itself due to changes in environmental conditions. For example, when the oscillation wavelength shifts at random between the neighboring modes in a single-mode laser, intensity of the laser light varies depending on the difference in gain of the two modes, thereby causing noise. The difference in gain of the modes increases as the interval between the modes increases, and then the relative intensity noise (RIN) is also increased. When the mode intervals are very large, however, the shift of the mode is unlikely to occur, since the mode that can obtain the gain is restricted, in which case occurrence of the mode-hopping noise is suppressed. That is, RIN becomes very large when the difference in gain between the modes is relatively large but the interval therebetween is not large enough to suppress occurrence of the mode shift, compared to the case of mode-hopping at the minimum Fabry-Perot mode interval.
On the other hand, it is known that there is a problem of noise due to optical feedback when a semiconductor laser is used for a light source of an optical information recording apparatus. In particular, with a laser of high coherency such as a single-mode oscillation laser, RIN tends to increase considerably, and errors are likely to occur at the time of recording or reading information on an optical disk or the like. It is reported that the coherency and RIN of a GaAs-based laser or the like can be lowered by utilizing high-frequency modulation or self pulsation to cause longitudinal multi-mode oscillation and then such an improved laser is suitable for a light source of an optical information recording apparatus. With the GaN-based laser having strong mode selectivity as shown in FIG. 20, however, ripples are present on the spectrum, and thus it is difficult to lower the coherency and RIN even with high-frequency modulation or self pulsation.